Roof Square Footage Calculator: How Big Is My Roof?

Roof square footage is the area of the sloped surface you actually cover with shingles, metal or membrane. It’s always larger than the footprint of the house because the roof is tilted, and roofers price and order it in squares of 100 sq ft. You don’t need to climb up to measure it: the outside dimensions of the house, the overhangs and the pitch are enough.

How it works

The calculator starts with the plan area, the flat footprint the roof covers when you look down on it. For a rectangular house that’s the wall dimensions plus the overhang on each side. Then it tilts that footprint to the roof’s pitch:

plan area   = (length + 2 × rake overhang) × (width + 2 × eave overhang)
multiplier  = √(1 + (pitch ÷ 12)²)
roof area   = plan area × multiplier
squares     = roof area ÷ 100

The pitch multiplier is how much longer a sloped line is than its level shadow. On a 6/12 roof every foot of level run becomes 1.118 ft of roof, so every square foot of footprint becomes 1.118 sq ft of roof. Because a single pitch applies in both directions, this rule works the same way for gable, hip and shed roofs. A hip roof with the same pitch and overhangs has exactly the same area as a gable roof over the same footprint.

If you already know the footprint area, for example from a site plan or an aerial view, choose Footprint area under the roof and skip the dimensions. Make sure the figure includes the overhangs.

The waste factor adds material for cuts, the starter course and ridge cap. The calculator rounds the result up to the next third of a square, since most shingles are sold three bundles per square.

When you enter dimensions, you also get edge lengths. Eaves are the level bottom edges where gutters and drip edge go. Rakes are the sloped edges on gable ends, which also get drip edge. Ridge and hip lengths tell you how much ridge cap to buy. On a hip roof, the ridge is the long side minus the short side, and each of the four hips runs from a corner up to the ridge at a little over 45° in plan.

Assumptions: the roof has one pitch on all its planes and sits on a rectangle. For an L-shaped house, a garage with its own roof, or a porch at a lower pitch, run each section separately and add the results.

Worked example

Take a single-story ranch, 52 ft long and 30 ft wide, with a hip roof at 5/12 and an 18-inch overhang all the way around.

• Plan area: (52 + 3) × (30 + 3) = 55 × 33 = 1,815 sq ft
• Multiplier: √(1 + (5 ÷ 12)²) = √1.1736 = 1.0833
• Roof area: 1,815 × 1.0833 = 1,966 sq ft, or 19.66 squares
• With 15% waste for a hip roof: 1,966 × 1.15 = 2,261 sq ft, which rounds up to 22⅔ squares, or 68 bundles

For trim, the eaves run the whole perimeter: 2 × (55 + 33) = 176 ft of drip edge and gutter line, with no rakes. The ridge is 55 − 33 = 22 ft. Each hip is 16.5 × √(2 + 0.174) = 24.3 ft, so the four hips add 97.3 ft of hip cap. Together that’s about 119 ft of ridge and hip cap.

If the same house had a gable roof, the area would still be 1,966 sq ft. You could drop the waste to 10%, and you’d trade the hips for two gable ends with 71.5 ft of rake edge.

Roof area reference table

Roof area per 1,000 sq ft of footprint. Scale it to your house: a 1,500 sq ft footprint at 6/12 is 1.5 × 1,118 = 1,677 sq ft of roof.

Tips and common mistakes

Don’t start from the listing square footage. A house’s advertised size is living area, and it counts both floors of a two-story house. It ignores garages and porches and leaves out overhangs. Measure the outside walls or use the footprint from a site plan.

Count the overhangs. On a typical house, a 12- to 18-inch overhang adds 10 to 20 percent to the footprint. It’s the most common reason homeowner estimates come in short.

Get the pitch right. An error of one pitch step moves the area by about 3 to 4 percent at common pitches. If you don’t know it, measure from the attic with a level and a tape, then use our roof pitch calculator.

Split complex roofs into rectangles. Break an L or T shape into rectangles that don’t overlap, and run each one at its own pitch. Count the overhang only on the edges that are actually exposed. A valley where two sections meet doesn’t add area, but it does add waste and needs valley flashing.

Treat the waste factor as a range. Simple gables waste less than 10% with careful layout, and roofs with lots of valleys, dormers and skylights can need 20%. Ask your supplier about their return policy on full bundles before you round up generously.

Measure from the ground or the attic. You can get everything this calculator needs without climbing on the roof. If you do go up, use fall protection. Falls are one of the leading causes of death in construction.